1. Field of the Invention
This invention relates in general to settable wellbore tools, and in particular to settable wellbore tools which are carried into wellbores on either coiled-tubing strings or wirelines and subjected to pressure differentials during setting.
2. Description of the Prior Art
Recent advances in the technology relating to the work-over of producing oil and gas wells have greatly enhanced the efficiency and economy of work-over operations. Through the use of either a coiled-tubing string, or a wireline assembly, work-over operations can now be performed through the production tubing string of a flowing oil and gas well. Two extremely significant advantages have been obtained by the through-tubing technology advances. First, the production tubing string does not need to be removed from the oil and gas well in order to perform work-over operations. This is a significant economic advantage, since work-over rigs are expensive, and the process of pulling a production tubing string is complicated and time consuming. The second advantage is that work-over operations can be performed without "killing" the well. As is known by those in the industry, the "killing" of a producing oil and gas well is a risky operation, and can frequently cause irreparable damage the worked-over well. Until the recent advances in the through-tubing work-over technology, work-over operations usually required that the well be killed.
While the through-tubing work-over technologies have significant advantages, some operating problems have been encountered. One problem, which is addressed by the present invention, is that pressure differentials created within the wellbore by the flowing wellbore fluids can cause unintended displacement of settable wellbore tools, such as bridge plugs and packers. Flow in either direction can exist in a wellbore if a producing zone is in hydraulic communication through the wellbore with a consuming zone. Such interzonal "cross-flow" may exist in a well irrespective of whether it is flowing to the surface.
Settable wellbore tools are operable in a plurality of operating modes including running modes of operation, expansion modes of operation, and setting modes of operation. The settable wellbore tool is maintained in a running condition during a running mode of operation, with a reduced radial dimension so that the settable wellbore tool may be passed downward into the oil and gas well through the production tubing. Once the settable wellbore tool is passed beyond the lower end of the production tubing string, and placed in a desired location, force is applied to the settable wellbore tool to urge it into an expansion mode of operation in which the wellbore tool is urged radially outward from a reduced radial dimension to an intermediate radial dimension, which at least in-part obstructs the flow of wellbore fluid within the wellbore in the region of the settable wellbore tool.
The obstruction created by the settable wellbore tool frequently creates a pressure differential across the settable wellbore tool. Most commonly, this occurs when a packer or bridge plug is set above a producing zone. Wellbore fluids, such as oil and water, will continue flowing into the well due to the pressure differential between the wellbore fluids in the earth's formation and the wellbore itself, as well as the pressure differential between different zones. Consequently, the wellbore fluids tend to flow within the well. However, the settable wellbore tool at least in-part obstructs the flow of wellbore fluids, and, consequently, a pressure differential is created across the wellbore tool.
The cross flow of fluids may urge the settable wellbore tool upward within the wellbore, away from the desired setting location. This unintended, and harmful displacement of the settable wellbore tool can occur because the new through-tubing, work-over technologies do not provide suspension means which are as "stiff" as those found in the more conventional work-over technologies. For example, a wireline-suspended, through-tubing work-over tool offers little resistance to pressure differentials which operate to lift the settable wellbore tool in position within the wellbore. Coiled tubing suspension means also do not provide sufficient "stiffness" to upward movement of the settable wellbore tool.
Additionally, if a pressure differential is developed across the settable wellbore tool with a higher pressure level above the settable wellbore tool, the pressure differential may act to disconnect the settable wellbore tool from the suspension means. In a wireline suspended, through-tubing wellbore tool a sufficiently large pressure differential could snap the wellbore tool loose from the wireline cable. Alternately, a high pressure differential could serve to accidentally actuate pressure-sensitive, or tension sensitive, disconnect devices which are used in both wireline-suspended tools and coiled-tubing suspended tools.